The number of Global Positioning System (GPS) devices has grown rapidly in recent years among military, civil and commercial users around the world. GPS is a space-based satellite navigation system that provides location and time information. In general, GPS can function anywhere on or near the Earth where there is an unobstructed line of sight to a plurality of GPS satellites. In some situations, GPS receivers can locate themselves in three spatial dimensions and time where there is an unobstructed line of sight four or more GPS satellites. However, GPS based position information may not be accessible in canyons or mountainous areas, in urban regions with many buildings, when there is an outage of a GPS satellite, inside buildings, subterranean locations, or underwater. GPS has a variety of uses on land, at sea, and in the air for both commercial and military applications. For example, GPS may facilitate activities such as banking, mobile phone operations, navigation of commercial or military aircraft and boats, scientific studies, tracking, and surveillance, etc.
The GPS can include three major segments: a space segment (SS), a control segment (CS), and a user segment (US). The United States Air Force develops, maintains, and operates the space and control segments for GPS activities in the United States. GPS satellites broadcast signals from space, and each GPS receiver uses these signals to calculate a three-dimensional location (latitude, longitude, and altitude) of the GPS receiver and a current time at each GPS satellite. The space segment can include a constellation of 24 to 32 satellites in a medium Earth orbit (MEO). The control segment can include a master control station, an alternate master control station, and a host of dedicated and shared ground antennas and monitor stations.
Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.